upfluence/http/server: add dedicated admin server to prevent thread starvation

[AlexisMontagne]

In our current setup, Puma has a limited thread pool shared between all incoming requests — unlike our Go servers. This creates two compounding problems during traffic spikes or slow request periods:

• Healthcheck endpoints get starved, causing pods to fail their liveness probes and enter a crash loop spiral
• The Prometheus scrape endpoint becomes unreachable during starvation, blinding alerting exactly when it is needed most

This PR introduces an optional admin server (env: ADMIN_PORT) that binds a separate Puma listener exclusively for control plane traffic. The production builder handles application requests; the admin builder handles /healthcheck, the Prometheus exporter, the Thrift base processor, and /debug. The two are fully isolated — a saturated production thread pool has no impact on admin traffic.

• Add admin_port option (env: ADMIN_PORT); when set, spins a side thread binding the admin builder on that port
• Mount /healthcheck, prometheus exporter, base thrift processor, and /debug on the admin builder; keep the production builder clean
• Fix spec hang: replace require 'sinatra' with require 'sinatra/base' in both api_endpoint files — sinatra/main registers an at_exit hook that starts a WEBrick server and blocks process exit
• Add server_spec.rb covering both builders, healthcheck, debug mount, and custom healthcheck endpoint